CF Industries Holdings Inc. has rejected rival fertilizer maker Agrium Inc.'s sweetened takeover bid, saying the $4.5 billion bid significantly undervalues the company.

CF Industries, which is based in Deerfield, Ill., rejected the latest bid in a statement on Friday, a day after its Canadian rival had raised its unsolicited offer.

Agrium has pursued CF since February, but has faced repeated rejections.

On Thursday, Agrium raised its bid to $45 in cash plus one Agrium share for each CF share. That's a $5 increase in the cash portion of the proposal. The total value of the deal is $92.99 per CF share, or $4.52 billion, based on CF outstanding shares as of Sept. 30.

Agrium had repeated what it has said in previous offers _ that its latest bid was its "best and final offer."

The fertilizer industry's hunger for consolidation intensified at the start of the year when company share prices had been beaten down to significantly low levels. As the economy declined, prices for agricultural commodities tumbled, forcing struggling farmers to scale back demand for fertilizer. As demand and pricing suffered, so did stocks of fertilizer companies.

Share prices have since recovered with an improving outlook for the industry, but still remain far below last year's levels.

CF started the acquisition trend in January, when it began its pursuit of another rival Terra Industries Inc., based in Sioux City, Iowa.

CF's advances continue to face resistance.